I have searched on google for it, and get conflicting answers.

I know chests, safes, bookshelves and wardrobes are safe, but what about barrels and sacks?

More specifically, the ones in the house on Raven Rock.

There's no place you can be sure is truly safe other than your purchased properties, all though I do believe the Raven Rock house you get has safe storage. I use chests, safe, and more in there but I have never used barrels, so not sure. I'd try putting a few items in leaving them for about a month in game time and see if they're still there after traveling.

I love your avatar.

You can store thing in barrels if they are empty, but if you find food in them, they replenish after some time, deleting whatever you put there. In Breezehome you have an empty barrel behind the stair. There are more in heartfire houses (main hall), but some contain food. Those are not safe to store your items.